Scientists for the first time in history managed to obtain a Wigner crystal, consisting only of electrons
For the first time in history, engineers at ETH Zurich managed to obtain a real crystal, which consists exclusively of electrons. The so-called Wigner crystals were theoretically predicted 90 years ago, but only now they were able to be observed live directly in a semiconductor material.
How it was possible to create and observe a crystal of electrons
Under normal conditions, the behavior of electrons resembles the behavior of a liquid that flows freely through a material. But already back in 1934, theoretical physicist Yu. Wigner formulated a theory according to which a group of electrons is quite capable of crystallizing into a solid form, forming a phase that is now referred to as the Wigner crystal.
So, according to the theory, for this you need to "catch" the ideal balance between forces such as electrostatic repulsion and the energy of motion.
So the energy of motion is a significantly more powerful factor that makes electrons bounce in a variety of directions. But if this force could be reduced (according to Wigner's assumption), then the repulsive force would have a stronger effect on the electrons and, thus, would lock them in a homogeneous lattice.
So over many decades, various groups of engineers have tried to confirm Wigner's theory and create a crystal consisting of electrons, but this turned out to be a rather difficult task.
After all, for this you need to reduce the density of electrons. In addition, they need to be fixed in a "trap", and also cooled to a temperature close to absolute zero in order to minimize the influence of external factors on them.
How the Wigner crystal was obtained
And only scientists from ETH Zurich managed to meet all the requirements for obtaining a Wigner crystal. So to confine electrons, a monatomic sheet of molybdenum diselenide was used, which effectively limited electrons to two dimensions.
To control the number of electrons, engineers clamped this material between two graphene electrodes and applied a minimum voltage. And so this structure was cooled to almost absolute zero.
So, as a result of such manipulations, the Wigner crystal appeared. But this turned out to be only half the battle, because the distance between the electrons turned out to be so small (about 20 nanometers) that it was impossible to see the crystal with a microscope.
To visualize the crystal, the scientists decided to apply a new method. It was decided to direct a stream of light onto the material with a fixed frequency in order to start the process of excitation of the so-called "excions" in the semiconductor, which emit light back.
If Wigner crystals are present, then the extions should appear stationary when they reflect light back.
Moreover, this effect should manifest itself in the observed excitation frequencies of excions, and this is precisely what scientists observed during their experiment to obtain a Wigner crystal.
Scientists have shared the results of the work done on the pages of the journal Nature.
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